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I have watched the videos, read the articles, attempted to understand the math.....but can't wrap my brain around accepting this "fact".

I'm with you John. It seems to me that after you reach wind speed, any faster and you would be going into the wind. A hot air balloon travels at the same speed as the wind, therefore there is zero speed relative to the wind in which it is traveling. It seems to me that the power supply would go away as soon as you reach wind speed, if you are going directly downwind.

Now, across the wind, sailors call it a reach, faster than the wind is easy. A sport kite flying is always on a reach as it is held to one spot relative to the to the wind, causing the wind to propel it.

I am not from Missouri, however, I think I would have to see this in person, not on a video.

I was a total skeptic at first too, after reading and watching I got a bit of a grasp on the concept. Could I explain it? Not easily. Do I believe it? Frankly I'm still skeptical. Could the added thrust of that low tech propeller overcome the drag induced by tapping power off the wheels, the drag of the mechanical transfer of power from them to the propeller, plus the rotational drag of the propeller? It's still deep in the realm of perpetual motion. You CAN NOT get more energy out of something then the energy put into it or stored in it. They pushed the vehicle up to 14 mph, but I don't see enough mass in the system to have stored that energy and returned it after the truck backed away.

But then you can sail faster then the wind speed at an angle to the wind. The only way this works is that the angled and spinning propeller actually ends up acting as a sail at an angle to the wind, I'm guessing that's what all the charts and graphs are trying to explain. The added thrust of creating an environment where the sail is angled to the direction of the wind is adding enough thrust to overcome the added drag. See the speedkiting thread for relevant information from our own sport, they are getting a better then 4 to 1 increase in speed over wind speed, so 2.8 to 1 doesn't sound unreasonable. I suppose we'll be seeing giant rotating sails and water wheels on the next America's Cup racers.

« Last Edit: November 15, 2010, 10:38 PM by mikenchico »

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"Living is easy with eyes closed, misunderstanding all you see" John W Lennon

"People do not quit playing because they grow old, they grow old because they quit playing" George Bernard Shaw

I think I'm convinced, though I'm still having a little trouble imagining it accelerating through the patch where it's going right AT the wind-speed, directly downwind -- essentially through still air! But if it can get there, above the wind-speed, I'm guessing that it really does work, without violating the laws of Physics.

It's true, and well known, that an efficient sailboat (especially a "land yacht" or an iceboat) "tacking downwind" -- zig-zagging downwind at around 45 degrees to the wind -- can not only go faster than IT could go straight downwind, but can also GET downwind faster than the wind does. It seems nuts, but it definitely works. And sailing UPwind, by zigzagging at around 45 degrees to the wind, also seems nuts, but it also works, and is used by sailors all the time. So the only questions are (1) whether a propeller can be used to "sail" ACROSS the wind in such a way that it propels a wheeled cart upwind and downwind (sure, why not?), and (2) whether such a craft could continue to accelerate straight downwind right up to the wind-speed, and through it.

I'm still having more trouble with #2 than #1, as follows: IF this cart can accelerate through that "speed = wind-speed barrier" -- when the "apparent wind" (the speed of the wind passing over the cart) is clearly ZERO -- then why couldn't it also run indoors, in no wind, accelerating from a dead stop? WELL, you say, the difference is that in the first example (in the wind) the propellor is already moving at a good clip, which generates apparent wind ON THE PROPELLOR BLADES, which isn't there indoors at a stop. OK, say I, then how about giving it a shove indoors, so the cart and the propellor both have some speed and momentum? Surely you can't believe that it will then start accelerating?!?

But I DO believe that a geared prop-cart could be made to creep upwind -- and that going upwind would increase the power the prop generates, so it would probably accelerate (like a sailboat tacking upwind) until (I guess) the wind resistance and friction became too great to accelerate any faster(?). And if that works straight upwind, then it's not obvious why it doesn't work indoors with a "jump start" -- except that I'm pretty sure that DOES violate the laws of Physics. . .

I'm still having more trouble with #2 than #1, as follows: IF this cart can accelerate through that "speed = wind-speed barrier" -- when the "apparent wind" (the speed of the wind passing over the cart) is clearly ZERO

-- then why couldn't it also run indoors, in no wind, accelerating from a dead stop? WELL, you say, the difference is that in the first example (in the wind) the propellor is already moving

The key difference is that in the first example you have a velocity difference between air and ground (true wind). In the second example air and ground have the same relative speed to the cart, even if you shove it.

For me, the key to understanding was thinking abut what happens at a dead stop. The craft goes forward just because its "entire self" is being pushed forward by the wind. As it goes forward, the wheels roll, which turn the propeller, which gives it an additional forward boost. The propeller is never being turned by the wind, it is spinning against the wind.

The craft is getting energy from the differential between the air speed over the ground, which is independent of its own velocity. Equilibrium speed is at the point where air resistance (from the non-propeller bits of the craft that are going faster than wind) and friction/torque (from driving the spinning propeller) balance the force generated by this differential.

Ever heard of dynamic soaring ? Even after the explanation and many, many videos I was still going "no, but wait.." 'cos it's just counter intuitive. To me This seems about the same.

Mike.

Never had, interesting and those were some incredible speeds. I'll have to study some pages on that to try to understand it, but I can see where there is an addition of energy due to moving between differing wind speeds. That's where gaining speed over the wind in direct downwind sailing makes my head hurt, no addition of energy. I had theorized in the first discussion that the rotating prop worked like a sail going across the wind and we know you can go faster than the wind speed but that's not directly downwind, one of the animations above seems to agree with that.

Going to have to look more into that dynamic soaring, pretty wild.

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"Living is easy with eyes closed, misunderstanding all you see" John W Lennon

"People do not quit playing because they grow old, they grow old because they quit playing" George Bernard Shaw

It is in fact similar. In DS you exploit the velocity difference between two air-masses, to achieve speeds higher than that difference. In DDWFTTW you exploit the velocity difference between ground and airmass (true wind), to achieve speeds higher than that difference.

I had theorized in the first discussion that the rotating prop worked like a sail going across the wind and we know you can go faster than the wind speed but that's not directly downwind, one of the animations above seems to agree with that.

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